Method for controlling lean combustion stability

ABSTRACT

A method for eliminating or substantially reducing unstable combustion conditions within a combustion chamber. By adding hydrogen to the inlet fuel mixture, the method provides a means for altering the overall combustion (reaction) rate and the resulting characteristic reaction time that, in turn, will eliminate or substantially reduce undesirable pressure oscillations. Moreover, the invention provides a low cost and easily installed method for active control of pressure oscillations within a combustion chamber

STATEMENT OF GOVERNMENT INTEREST

[0001] This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] Not applicable.

FIELD OF THE INVENTION

[0003] The invention pertains generally to a method for substantially reducing or eliminating unstable combustion conditions, including pressure oscillations, within a combustion chamber, in particular, unstable conditions resulting from ultra lean premixed combustion.

BACKGROUND OF THE INVENTION

[0004] The development of low-emission, high performance combustors is an area of considerable current interest. In particular, ultra lean premixed combustion is currently recognized as an effective approach to reduction of nitrogen oxides (NOx) emissions from a variety of combustion sources, including gas turbines for power generation and aircraft propulsion as well as a variety of boilers, furnaces, heaters, and other combustion devices. Unfortunately, the use of lean premixed combustion is often limited by the onset of combustion instabilities. These instabilities are generally understood to be high amplitude pressure oscillations in the combustion chamber that are driven by combustion heat release. Under the right conditions, the amplitude of these pressure fluctuations increases and can, at a minimum, result in a degradation of combustor performance. In the limit, the amplitude of the pressure fluctuations can be sufficient to cause significant damage to combustor hardware and burner components. Whether the combustor operates in a stable mode or an unstable mode is determined by numerous factors. These can include, but are not limited to, fuel type, fuel/air ratio, inlet pressure, combustor geometry, combustor throughput, and the coupling between combustion chamber design and flame heat release.

[0005] Combustion consists of a chemical reaction between a mixture of fuel and air to release heat. The term equivalence ratio is often used to identify the actual quantities of fuel and air provided. As used herein, the term is defined as the ratio of fuel to air provided divided by the stoichiometric ratio of fuel to air. The stoichiometric ratio is achieved when the proper amount of air is provided to completely consume all the fuel. Thus, an equivalence ratio of unity corresponds to an amount of air exactly equal to that needed to consume all the fuel while an equivalence ratio less than unity indicates excess air, i.e., a fuel lean condition. Typically maximum combustion temperatures occur at near stoichiometric conditions (near an equivalence ratio of unity). As the equivalence ratio exceeds or becomes less than unity the combustion temperature decreases. NOx emissions are a strong function of temperature, increasing exponentially with increasing temperature. Thus, lean premixed combustion is an effective control strategy for NOx emissions because the combustion temperature can be decreased by the addition of excess air. However, this control strategy is limited by the finite operating range of a combustor. At sufficiently fuel-lean conditions (typically near an equivalence ratio of 0.5) the flame temperature becomes sufficiently low that the heat loss rate exceeds the combustion heat release rate and the flame can no longer sustain itself. This condition is referred to as the lean blowout limit and provides a lower boundary for the fuel/air mixture ratio.

[0006] Combustion instabilities occur over a finite range of combustor operating conditions. These instabilities are the result of a complex interaction between combustion heat release and acoustic pressure waves that exist within the combustor volume. Operation under fuel-lean conditions, which is desirable from the point of view that NOx emissions will be decreased, is particularly vulnerable to these instabilities since small perturbations in temperature and pressure have large effects on the combustion heat release rate, which promotes coupling between heat release and acoustic instabilities.

[0007] Approaches for removing combustion instabilities include both passive and active control methods. Current passive control techniques can include modification of the fuel injection system to modify the fuel distribution and resulting heat release distribution pattern in the combustion chamber, modifying the air injection system through changes in geometry to modify fuel/air mixing patterns or modifications to the combustion chamber itself, either through variations in its size or shape. These modifications can be costly and can limit or degrade combustion and system performance.

[0008] Active controls typically monitor real-time combustor performance with regard to a variable of interest selected to best characterize performance. The results of this monitoring provide the input needed to actively modify the combustor environment and remove undesirable performance characteristics such as pressure oscillations and instabilities. Current active control strategies include feedback to actively control the fuel or air flows so as to avoid operating conditions that promote instabilities. Often this involves high frequency cycling or modulation of the fuel or air flow rate to change the phase of combustion heat release with respect to the undesired oscillations and thus remove the coupling between the heat release and oscillations driven by it. Disadvantages of active control techniques are the complexities associated with detecting and monitoring combustion chamber stability, modulating the fuel and/or air flows, and obtaining the knowledge needed to effectively modulate these input parameters to eliminate or control instabilities. These complexities arise because of the complex interaction and coupling between inlet flow rates, mixing, and where in the combustion chamber combustion is initiated.

[0009] It has been shown (Keller, Combustion and Flame, 75, 33-44, 1989) that the phase relationship between energy release and the resonant pressure wave is controlled by the relative magnitudes of the total ignition delay time and the characteristic acoustic resonance time. Further, to a first order approximation, the total ignition delay time is determined by, amongst other parameters, the time for a chemical reaction to occur. Consequently, the addition of a fuel that possesses a higher reaction rate than typical fuels, such as natural gas or methane, would alter the overall chemical reaction rate and the resulting characteristic reaction time, thereby providing a means for tuning the combustion process to be “tuned” in or out of phase with the resonant pressure waves.

SUMMARY OF THE INVENTION

[0010] Accordingly, the present invention is directed to a low cost, easily installed method for active control of unstable combustion, namely pressure oscillations, within a combustion chamber comprising the addition of hydrogen to the inlet fuel mixture. Hydrogen has a significantly higher reaction rate than typical hydrocarbon fuels (typically a factor of five grater than the corresponding rate for natural gas and common hydrocarbon fuels under comparable conditions). Thus, the basis of the method is the controlled addition of hydrogen gas to a base fuel mixture in a combustor to alter the overall combustion (reaction) rate and the resulting characteristic reaction time.

[0011] Specifically, the method involves varying the ratio of conventional or base fuel to hydrogen in the inlet fuel mixture, comprising a base fuel and sir. The base fuel can be natural gas, however, the process is equally applicable to other common fuels such as methane, coal gas, biomass-derived fuels, or other hydrocarbon fuel materials. The method is particularly applicable, but not limited to, a combustor operating under fuel-lean premixed conditions, a desirable operating condition to reduce emissions of nitrogen oxides (NOx). The unique combustion characteristics of hydrogen sufficiently alter the burner combustion characteristics to allow operation at the desired conditions without being limited by the onset of combustion instabilities.

[0012] In FIG. 1 the total flow rate of fuel and air (inlet fuel mixture) into the combustor is plotted against the equivalence ratio (the ratio of fuel to air provided divided by the stoichiometric ratio of fuel to air) with the unstable operating region indicated by the shaded area. Also shown in FIG. 1 is the lean blowout limit line; combustion cannot be sustained at equivalence ratios to the left of this line.

[0013] Operating conditions in a combustor are normally dictated by power requirements. Additionally, combustion temperatures, which determine the combustor operating efficiency, can be limited by outlet conditions, such as maximum blade temperature, as well as NOx emission considerations. Consequently, operational factors that dictate fuel flow rate and equivalence ratio can require operating a combustor in an unstable operating region, (FIG. 1a) that, as discussed above, can lead to degradation in combustor performance and potentially damaging pressure oscillations.

[0014] The effect of proper hydrogen addition on the unstable operating region is shown in FIG. 1b. Here, the faster chemical reaction times due to hydrogen addition result in a shift of the instability island to the right of the operating point. This shift allows operation at the desired combustor flow rate and equivalence ratio. An added benefit is that for hydrogen addition up to about 40%, the flame temperature remains nearly the same for a fixed equivalence ratio. Consequently, the location of the instability region can be controlled while maintaining the same equivalence ratio and flame temperature. This is particularly advantageous since combustor performance and emissions are closely related to flame temperature.

[0015] An instability mode typical of the operation of a lean premixed fuel combustor is shown in FIG. 2. In this case, the instability region is bounded on one side by the lean blowout limit line. For no hydrogen addition (FIG. 2a) the desired operating point is located within the unstable operating region. For a lean premixed fuel combustor, proper selection of the amount of hydrogen addition shifts the boundary of the unstable region away from the desired operating point, thereby allowing stable operation at the desired flow rate and equivalence ratio.

[0016] Thus, the invention provides the advantage that a combustor can be operated at an operating point (equivalence ratio and total fuel flow rate) dictated by required operating conditions by using hydrogen addition to minimize combustion oscillations. This is an advantage over other active control techniques that require operating conditions to vary cyclically from the desired operating point to minimize combustion oscillations.

[0017] The invention provides additional advantage in that it can be embodied in multiple-injector devices having more than one fuel/air injection nozzle. In this case, the method can be applied to each fuel injector individually or to selected groups of fuel injectors. In this way the invention can be applied to separate combustion zones that can require different hydrogen additions to minimize oscillations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 shows a relationship between a combustor operating point and the unstable operating region without (FIG. 1a) and with (FIG. 1b) hydrogen addition.

[0019]FIG. 2 shows a relationship between a combustor operating point and the unstable operating region for a lean premixed combustor without (FIG. 2a) and with (FIG. 2b) hydrogen addition.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention is directed to a novel method for actively controlling unstable combustion conditions, including pressure oscillations, that can occur within a combustion chamber or combustor by providing an inlet fuel mixture comprising a base hydrocarbon fuel that can be a natural gas, methane, coal gas, biomass-derived fuel, or other hydrocarbon fuel materials, air, and hydrogen, wherein the hydrogen is in sufficient concentration to eliminate pressure oscillations. Hydrogen addition is used to control (dampen) the amplitude of combustion oscillations at the desired operating point or to shift the region of combustion oscillations away from the desired operating point. The rate of addition of the inlet fuel mixture is controlled to maintain a nearly constant fuel/air equivalence ratio and thereby constant combustion temperature and energy throughput rate.

[0021] In one embodiment of the invention, determination of combustor stability characteristics in a multi-dimensional space defined by fuel flow rate, equivalence ratio, and hydrogen concentration is required. In practice, combustor stability characteristics are determined as a function of hydrogen concentration for a given value of inlet fuel flow rate and equivalence ratio to produce graphs such as those illustrated in FIGS. 1 and 2. As will be appreciated by those skilled in the art, these characteristics can be determined using a test burner consisting of either single fuel/air injectors, a sectional cut of multiple-injector configurations, or using an entire combustion section. Once the stability characteristics have been determined, comparison with desired operating points provide the parameters necessary to define hydrogen addition requirements necessary to promote stable operation.

[0022] In a second embodiment of the present invention, the pressure within a combustion chamber is measured by pressure sensing devices communicating with the combustion chamber. Whenever the amplitude of pressure fluctuations exceeds a predetermined threshold value for a predetermined period of time, hydrogen is added to the inlet fuel mix in amounts predetermined to eliminate or substantially reduce pressure oscillations. Threshold values and duration times are selected based on material and design strength considerations to be below levels that would cause damage. 

We claim:
 1. A method for active control of combustion oscillations within a combustion chamber, comprising the step of: adding hydrogen to the inlet fuel mixture provided to the combustion chamber in an amount sufficient to eliminate pressure oscillations.
 2. The method of claim 1, wherein the inlet fuel mixture contains a base hydrocarbon fuel including natural gas, methane, coal gas, biomass-derived fuel, or other hydrocarbon fuel materials
 3. A method for eliminating unstable combustion conditions within a combustion chamber, comprising the steps of: determining the stability characteristics of the combustion chamber as a function of hydrogen concentration for a given value of inlet fuel flow rate and equivalence ratio; establishing an operating point for the combustor; and adding hydrogen to the inlet fuel mixture provided to the combustion chamber in an amount sufficient to eliminate pressure oscillations.
 4. The method of claim 3, wherein the inlet fuel mixture contains a base hydrocarbon fuel including natural gas, methane, coal gas, biomass-derived fuel, or other hydrocarbon fuel materials
 5. A method for the active control of pressure oscillations within a combustion chamber, comprising: measuring the amplitude of pressure fluctuations within a combustion chamber; and providing a inlet fuel mixture comprising a base hydrocarbon fuel, air, and hydrogen to the combustion chamber, wherein the concentration of hydrogen is in an amount sufficient to eliminate or substantially reduce pressure fluctuations.
 6. The method of claim 5, wherein the base hydrocarbon fuel includes natural gas, methane, coal gas, biomass-derived fuel, or other hydrocarbon fuel materials 